Low-light conditions occur indoors, on overcast and foggy days, during storms and of course as darkness descends. It may seem natural to put your camera away under such circumstances but don't be tempted to do so. Excellent photographic opportunities are created as colours darken and merge together. Subtle tones dominate and the eye becomes more sensitive to small tonal gradations.
It is important to have a fast lens with a maximum aperture of f/2.8, f/2 or f/1.4, and to use the wide apertures to full effect despite depth of field limitations. Slow shutter speeds, 1/4 second for example, can also be used provided your subject does not move and the camera is mounted on a tripod.
After dark there are still lots of possibilities. Illuminate a face with a match, a candle or an oil lamp and wonderfully warm and intimate images are possible. Use a fast ISO setting in the range (400 - 6400+) and bear in mind the extreme contrast and low colour temperature. A suitable colour temperature setting, and/or later colour correction, will be required to achieve anything approaching perceived colour, and detail at one end of the brightness scale will probably be lost.